The Epidemiology Branch is conducting a number of birth defect studies in collaboration with the Health Research Board and Trinity College, Dublin, Ireland. The main objective of these studies is to determine the relationship between folate and birth defects. The birth defects studied to date are neural tube defects (NTDs), oral clefts, and Down syndrome. These studies focus on biochemical factors in the area of folate metabolism, and on genetic mutations in folate related genes associated with birth defects. In the past we have shown that elevated homocysteine is a risk factor for NTDs, that a mutation in the methylenetetrahydrofolate reductase (MTHFR) gene 677C->T is a risk factor for both NTDs and oral clefts, and that a small dose of folic acid (100-200 micrograms) can raise red cell folate to levels that can prevent a fifth to almost a half of NTDs. Recently, we have found that mothers of children with Down syndrome are significantly more likely to carry a variant of the gene for the enzyme methionine synthase reductase 66A->G, that is important in homocysteine metabolism. The risk of having a child with Down syndrome is further increased if the mothers have the MTHFR 677C->T variant as well. This risk appears to be due to altered folate metabolism in the embryonic period of the mother. We have also examined factors that influence fetal homocysteine levels. This investigation has shown that, in mothers receiving standard prenatal care including vitamins, maternal homocysteine levels are the strongest influence on fetal homocysteine levels. Both maternal and fetal vitamin B12 levels also influence homocysteine values significantly. Enzyme gene variants (MTHFR) and folate levels did not significantly influence fetal homocysteine, although the folate levels may have been high enough secondary to prenatal vitamins that they were not a factor. We are currently collecting data and samples for genetic analysis on subjects with oral clefts and their families at the major Dublin hospitals where cleft patients are managed. Our genetic studies are currently focused on looking for folate enzyme gene variants that are associated with NTDs.